P90 RSK arranges Chk1 in the nucleus for monitoring of genomic integrity during cell proliferation

Spatial organization and functions of Chk1 activation by TopBP1 biomolecular condensates

Assembly of TopBP1 biomolecular condensates triggers activation of the ataxia telangiectasia-mutated and Rad3-related (ATR)/Chk1 signaling pathway, which coordinates cell responses to impaired DNA replication. Here, we used optogenetics and reverse genetics to investigate the role of sequence-specific motifs in the formation and functions of TopBP1 condensates. We propose that BACH1/FANCJ is involved in the partitioning of BRCA1 within TopBP1 compartments. We show that Chk1 is activated at the interface of TopBP1 condensates and provide evidence that these structures arise at sites of DNA damage and in primary human fibroblasts. Chk1 phosphorylation depends on the integrity of a conserved arginine motif within TopBP1's ATR activation domain (AAD). Its mutation uncouples Chk1 activation from TopBP1 condensation, revealing that optogenetically induced Chk1 phosphorylation triggers cell cycle checkpoints and slows down replication forks in the absence of DNA damage. Together with previous work, these data suggest that the intrinsically disordered AAD encodes distinct molecular steps in the ATR/Chk1 pathway.

ALK signaling primes the DNA damage response sensitizing ALK-driven neuroblastoma to therapeutic ATR inhibition

High-risk neuroblastoma (NB) is a significant clinical challenge. MYCN and Anaplastic Lymphoma Kinase (ALK), which are often involved in high-risk NB, lead to increased replication stress in cancer cells, suggesting therapeutic strategies. We previously identified an ATR (ataxia telangiectasia and Rad3-related)/ALK inhibitor (ATRi/ALKi) combination as such a strategy in two independent genetically modified mouse NB models. Here, we identify an underlying molecular mechanism, in which ALK signaling leads to phosphorylation of ATR and CHK1, supporting an effective DNA damage response. The importance of ALK inhibition is supported by mouse data, in which ATRi monotreatment resulted in a robust initial response, but subsequent relapse, in contrast to a 14-d ALKi/ATRi combination treatment that resulted in a robust and sustained response. Finally, we show that the remarkable response to the 14-d combined ATR/ALK inhibition protocol reflects a robust differentiation response, reprogramming tumor cells to a neuronal/Schwann cell lineage identity. Our results identify an ability of ATR inhibition to promote NB differentiation and underscore the importance of further exploring combined ALK/ATR inhibition in NB, particularly in high-risk patient groups with oncogene-induced replication stress.

Endothelial activation and fibrotic changes are impeded by laminar flow-induced CHK1-SENP2 activity through mechanisms distinct from endothelial-to-mesenchymal cell transition

Background

The deSUMOylase sentrin-specific isopeptidase 2 (SENP2) plays a crucial role in atheroprotection. However, the phosphorylation of SENP2 at T368 under disturbed flow (D-flow) conditions hinders its nuclear function and promotes endothelial cell (EC) activation. SUMOylation has been implicated in D-flow-induced endothelial-to-mesenchymal transition (endoMT), but the precise role of SENP2 in counteracting this process remains unclear.

Method

We developed a phospho-specific SENP2 S344 antibody and generated knock-in (KI) mice with a phospho-site mutation of SENP2 S344A using CRISPR/Cas9 technology. We then investigated the effects of SENP2 S344 phosphorylation under two distinct flow patterns and during hypercholesteremia (HC)-mediated EC activation.

Result

Our findings demonstrate that laminar flow (L-flow) induces phosphorylation of SENP2 at S344 through the activation of checkpoint kinase 1 (CHK1), leading to the inhibition of ERK5 and p53 SUMOylation and subsequent suppression of EC activation. We observed a significant increase in lipid-laden lesions in both the aortic arch (under D-flow) and descending aorta (under L-flow) of female hypercholesterolemic SENP2 S344A KI mice. In male hypercholesterolemic SENP2 S344A KI mice, larger lipid-laden lesions were only observed in the aortic arch area, suggesting a weaker HC-mediated atherogenesis in male mice compared to females. Ionizing radiation (IR) reduced CHK1 expression and SENP2 S344 phosphorylation, attenuating the pro-atherosclerotic effects observed in female SENP2 S344A KI mice after bone marrow transplantation (BMT), particularly in L-flow areas. The phospho-site mutation SENP2 S344A upregulates processes associated with EC activation, including inflammation, migration, and proliferation. Additionally, fibrotic changes and up-regulated expression of EC marker genes were observed. Apoptosis was augmented in ECs derived from the lungs of SENP2 S344A KI mice, primarily through the inhibition of ERK5-mediated expression of DNA damage-induced apoptosis suppressor (DDIAS).

Summary

In this study, we have revealed a novel mechanism underlying the suppressive effects of L-flow on EC inflammation, migration, proliferation, apoptosis, and fibrotic changes through promoting CHK1-induced SENP2 S344 phosphorylation. The phospho-site mutation SENP2 S344A responds to L-flow through a distinct mechanism, which involves the upregulation of both mesenchymal and EC marker genes.

Maintaining Genome Integrity: Protein Kinases and Phosphatases Orchestrate the Balancing Act of DNA Double-Strand Breaks Repair in Cancer

DNA double-strand breaks (DSBs) are the most lethal DNA damages which lead to severe genome instability. Phosphorylation is one of the most important protein post-translation modifications involved in DSBs repair regulation. Kinases and phosphatases play coordinating roles in DSB repair by phosphorylating and dephosphorylating various proteins. Recent research has shed light on the importance of maintaining a balance between kinase and phosphatase activities in DSB repair. The interplay between kinases and phosphatases plays an important role in regulating DNA-repair processes, and alterations in their activity can lead to genomic instability and disease. Therefore, study on the function of kinases and phosphatases in DSBs repair is essential for understanding their roles in cancer development and therapeutics. In this review, we summarize the current knowledge of kinases and phosphatases in DSBs repair regulation and highlight the advancements in the development of cancer therapies targeting kinases or phosphatases in DSBs repair pathways. In conclusion, understanding the balance of kinase and phosphatase activities in DSBs repair provides opportunities for the development of novel cancer therapeutics.

P90 ribosomal S6 kinase confers cancer cell survival by mediating checkpoint kinase 1 degradation in response to glucose stress

In solid tumors, cancer cells have devised multiple approaches to survival and proliferate in response to glucose starvation that is often observed in solid tumor microenvironments. However, the precise mechanisms are far less known. Herein, we report that glucose deprivation activates 90‐kDa ribosomal S6 kinase (p90 RSK), a highly conserved Ser/Thr kinase, and activated p90 RSK promotes cancer cell survival. Mechanistically, activated p90 RSK by glucose deprivation phosphorylates checkpoint kinase 1 (CHK1), a key transducer in checkpoint signaling pathways, at Ser280 and triggers CHK1 ubiquitination mediated by SCF^β‐TrCP ubiquitin ligase and proteasomal degradation, subsequently suppressing cancer cell apoptosis induced by glucose deprivation. Importantly, we identified an inverse correlation between p90 RSK activity and CHK1 levels within the solid tumor mass, with lower levels of CHK1 and higher activity of p90 RSK in the center of the tumor where low glucose concentrations are often observed. Thus, our study indicates that p90 RSK promotes CHK1 phosphorylation at Ser280 and its subsequent degradation, which allows cancer cells to escape from checkpoint signals under the stress of glucose deprivation, leading to cell survival and thus contributing to tumorigenesis.

Reality CHEK: Understanding the biology and clinical potential of CHK1

The DNA damage response enables cells to cope with various stresses that threaten genomic integrity. A critical component of this response is the serine/threonine kinase CHK1 which is encoded by the CHEK1 gene. Originally identified as a regulator of the G2/M checkpoint, CHK1 has since been shown to play important roles in DNA replication, mitotic progression, DNA repair, and overall cell cycle regulation. However, the potential of CHK1 as a cancer therapy has not been realized clinically. Herein we expound our current understanding of the principal roles of CHK1 and highlight different avenues for CHK1 targeting in cancer therapy.

Phosphoglycerate Mutase Cooperates with Chk1 Kinase to Regulate Glycolysis

Dysregulated glycolysis, including the cancerous Warburg effect, is closely involved in pathological mechanisms of diseased states. Among glycolytic enzymes, phosphoglycerate mutase (PGAM) has been known to exert certain physiological impact in vitro, whereas its regulatory role on glycolysis remains unclear. Here, we identified that PGAM plays a key role in regulating glycolysis in cancer cells but not in standard cells. Cancer-prone phenotype by PGAM overexpression in vivo was associated with upregulated glycolytic features. PGAM interacts and cooperates with Chk1 to regulate the enhanced glycolysis in cancer cells, especially under oncogenic Ras expressing conditions. Genetic or chemical interference of the PGAM-Chk1 interaction, with intact PGAM activity, abrogated the maintenance of cancerous enhanced glycolysis. Thus, the nonenzymatic function of PGAM is essential for the Warburg effect that accompanies cancerous proliferation.

Chk1-mediated Cdc25A degradation as a critical mechanism for normal cell cycle progression

Chk1 (encoded by CHEK1 in mammals) is an evolutionarily conserved protein kinase that transduces checkpoint signals from ATR to Cdc25A during the DNA damage response (DDR). In mammals, Chk1 also controls cellular proliferation even in the absence of exogenous DNA damage. However, little is known about how Chk1 regulates unperturbed cell cycle progression, and how this effect under physiological conditions differs from its regulatory role in DDR. Here, we have established near-diploid HCT116 cell lines containing endogenous Chk1 protein tagged with a minimum auxin-inducible degron (mAID) through CRISPR/Cas9-based gene editing. Establishment of these cells enabled us to induce specific and rapid depletion of the endogenous Chk1 protein, which resulted in aberrant accumulation of DNA damage factors that induced cell cycle arrest at S or G2 phase. Cdc25A was stabilized upon Chk1 depletion before the accumulation of DNA damage factors. Simultaneous depletion of Chk1 and Cdc25A partially suppressed the defects caused by Chk1 single depletion. These results indicate that, similar to its function in DDR, Chk1 controls normal cell cycle progression mainly by inducing Cdc25A degradation.

SCFβ-TrCP ubiquitinates CHK1 in an AMPK-dependent manner in response to glucose deprivation

The ATR/CHK1 pathway is a key effector of cellular response to DNA damage and therefore is a critical regulator of genomic stability. While the ATR/CHK1 pathway is often inactivated by mutations, CHK1 itself is rarely mutated in human cancers. Thus, cellular levels of CHK1 likely play a key role in the maintenance of genomic stability and preventing tumorigenesis. Glucose deprivation is observed in many solid tumors due to high glycolytic rates of cancer cells and insufficient vascularization, yet cancer cells have devised mechanisms to survive in conditions of low glucose. Although CHK1 degradation through the ubiquitin-proteasome pathway following glucose deprivation has been previously reported, the detailed molecular mechanisms remain elusive. Here, we show that CHK1 is ubiquitinated and degraded upon glucose deprivation by the Skp1-Cullin-F-box (β-TrCP) E3 ubiquitin ligase. Specifically, CHK1 contains a β-TrCP recognizable degron domain, which is phosphorylated by AMPK in response to glucose deprivation, allowing for β-TrCP to recognize CHK1 for subsequent ubiquitination and degradation. Our results provide a novel mechanism by which glucose metabolism regulates a DNA damage effector, and imply that glucose deprivation, which is often found in solid tumor microenvironments, may enhance mutagenesis, clonal expansion, and tumor progression by triggering CHK1 degradation.

Chk1 KA1 domain auto-phosphorylation stimulates biological activity and is linked to rapid proteasomal degradation

The DNA damage-activated protein kinase Chk1 is known to undergo auto-phosphorylation, however the sites and functional significance of this modification remain poorly understood. We have identified two novel Chk1 auto-phosphorylation sites, threonines 378 and 382 (T378/382), located in a highly conserved motif within the C-terminal Kinase Associated 1 (KA1) domain. T378/382 occur within optimal consensus Chk1 phosphorylation motifs and substitution with phospho-mimetic aspartic acid residues results in a constitutively active mutant Chk1 kinase (Chk1-DD) that arrests cell cycle progression in G2 phase of the cell cycle in the absence of DNA damage. Remarkably, the mutant Chk1-DD protein is also subject to very rapid proteasomal degradation, with a half-life approximately one tenth that of wild-type Chk1. Consistent with this, T378/T382 auto-phosphorylation also accelerates the proteasomal degradation of constitutively active Chk1 KA1 domain structural mutants. T378/382 auto-phosphorylation and accelerated degradation of wild-type Chk1 occurs at low levels during unperturbed growth, but surprisingly, is not augmented in response to genotoxic stress. Taken together, these observations demonstrate that Chk1 T378/T382 auto-phosphorylation within the KA1 domain is linked to kinase activation and rapid proteasomal degradation, and suggest a non-canonical mechanism of regulation.

A PIM-CHK1 signaling pathway regulates PLK1 phosphorylation and function during mitosis

Although the kinase CHK1 is a key player in the DNA damage response (DDR), several studies have recently provided evidence of DDR-independent roles of CHK1, in particular following phosphorylation of its S280 residue. Here, we demonstrate that CHK1 S280 phosphorylation is cell cycle-dependent and peaks during mitosis. We found that this phosphorylation was catalyzed by the kinase PIM2, whose protein expression was also increased during mitosis. Importantly, we identified polo-like kinase 1 (PLK1) as a direct target of CHK1 during mitosis. Genetic or pharmacological inhibition of CHK1 reduced the activating phosphorylation of PLK1 on T210, and recombinant CHK1 was able to phosphorylate T210 of PLK1 in vitro Accordingly, S280-phosphorylated CHK1 and PLK1 exhibited similar specific mitotic localizations, and PLK1 was co-immunoprecipitated with S280-phosphorylated CHK1 from mitotic cell extracts. Moreover, CHK1-mediated phosphorylation of PLK1 was dependent on S280 phosphorylation by PIM2. Inhibition of PIM proteins reduced cell proliferation and mitotic entry, which was rescued by expressing a T210D phosphomimetic mutant of PLK1. Altogether, these data identify a new PIM-CHK1-PLK1 phosphorylation cascade that regulates different mitotic steps independently of the CHK1 DDR function.This article has an associated First Person interview with the first author of the paper.

Chk1 inhibition as a novel therapeutic strategy in melanoma

Melanoma patients respond poorly to chemotherapies because they acquire drug resistance. Therapies that can overcome the resistance to inhibitors of the mutated BRAF protein kinase in melanoma are urgently needed. Chk1 protein kinase is a central component of the DNA damage response and plays a crucial role in controlling cell cycle progression. Analyses indicate that low mRNA expression of Chk1 is significantly associated with good overall survival of melanoma patients. To evaluate the effectiveness of Chk1 inhibitors in melanoma therapy, we have generated BRAF inhibitor (PLX4032 or vemurafenib) resistant melanoma cell lines (A375-PLX-R and WM9-PLX-R) from A375 and WM9, respectively. We observe that AKT (protein kinase B) is constitutively activated in A375-PLX-R, but not in WM9-PLX-R cells, suggesting that these cells develop resistance to PLX4032 through different mechanisms. We show that a potent and specific inhibitor of Chk1 (PF477736) is effective in reducing cell viability and colony formation of PLX4032-resistant cells. Even more impressively, PF477736 triggers PLX4032-resistant melanoma cells to regain sensitivity to the PLX4032. Mouse xenograft studies show that treating A375-PLX-R derived tumors with combined PLX4032 and PF477736 significantly reduce tumor growth. Combined treatments with PLX4032 and PF477736 reduce the levels of total Chk1 protein and alter Chk1 phosphorylation at several sites in both PLX4032 sensitive and resistant melanoma cells. Combinatorial treatments with PLX4032 and PF477736 to melanoma cells substantially induce DNA damage and cell death. Our results suggest that Chk1 inhibitors may provide new therapy options for melanoma patients.

Evaluation of CHK1 activation in vulvar squamous cell carcinoma and its potential as a therapeutic target in vitro

CHK1 is an important regulator of the cell cycle and DNA damage response, and its altered expression has been identified in various tumors. Chk1 inhibitors are currently being evaluated as monotherapy and as potentiators of chemotherapy in clinical settings. However, to our knowledge, no previous study has investigated either the activation status or the therapeutic potential of CHK1 targeting in vulvar cancer. Therefore, we examined the expression status of activated CHK1 forms pCHK1^Ser345, pCHK1^Ser317, pCHK1^Ser296, and pCHK1^Ser280 in 294 vulvar squamous cell carcinomas (VSCC) using immunohistochemistry and analyzed their relationships with various clinicopathological variables and clinical outcome. To aid translation of preclinical studies, we also assessed cell sensitivity to the Chk1 inhibition in two vulvar cancer cell lines. Compared to the levels of pCHK1^Ser345, pCHK1^Ser317, pCHK1^Ser296, and pCHK1^Ser280 in normal vulvar squamous epithelium, high nuclear pCHK1^Ser345 expression was found in 57% of vulvar carcinomas, whereas low nuclear pCHK1^Ser317, pCHK1^Ser296, and pCHK1^Ser280 expressions were observed in 58%, 64%, and 40% of the cases, respectively. Low levels of pCHK1^Ser317 and pCHK1^Ser280 in the nucleus correlated significantly with advanced tumor behaviors and aggressive features. None of pCHK1^Ser345, pCHK1^Ser317, pCHK1^Ser296, and pCHK1^Ser280 forms were identified as prognostic factors. In vitro inhibition of CHK1 by small molecular inhibitors or siRNA reduced viability by inducing DNA damage and apoptosis of vulvar cancer cell lines. In summary, we conclude that cellular functions regulated by CHK1 are phosphorylation/localization‐dependent and deregulation of CHK1 function occurs in VSCC and might contribute to tumorigenesis. Targeting CHK1 might represent as a useful antitumor strategy for the subgroup of VSCC harboring p53 mutations.

The kinase domain residue serine 173 of Schizosaccharomyces pombe Chk1 kinase is critical for the response to DNA replication stress

While mammalian Chk1 kinase regulates replication origins, safeguards fork integrity and promotes fork progression, yeast Chk1 acts only in G1 and G2. We report here that the mutation of serine 173 (S173A) in the kinase domain of fission yeast Chk1 abolishes the G1-M and S-M checkpoints with little impact on the G2-M arrest. This separation-of-function mutation strongly reduces the Rad3-dependent phosphorylation of Chk1 at serine 345 during logarithmic growth, but not when cells experience exogenous DNA damage. Loss of S173 lowers the restrictive temperature of a catalytic DNA polymerase epsilon mutant (cdc20.M10) and is epistatic with a mutation in DNA polymerase delta (cdc6.23) when DNA is alkylated by methyl-methanesulfate (MMS). The chk1-S173A allele is uniquely sensitive to high MMS concentrations where it displays a partial checkpoint defect. A complete checkpoint defect occurs only when DNA replication forks break in cells without the intra-S phase checkpoint kinase Cds1. Chk1-S173A is also unable to block mitosis when the G1 transcription factor Cdc10 (cdc10.V50) is impaired. We conclude that serine 173, which is equivalent to lysine 166 in the activation loop of human Chk1, is only critical in DNA polymerase mutants or when forks collapse in the absence of Cds1.

Summary: Mutation of serine-173 in the kinase domain of Chk1 increases genomic instability as it abolishes the response to DNA lesions that arise while chromosomes are being copied.

Targeting AXL and mTOR Pathway Overcomes Primary and Acquired Resistance to WEE1 Inhibition in Small-Cell Lung Cancer

Purpose: Drugs targeting DNA repair and cell-cycle checkpoints have emerged as promising therapies for small-cell lung cancer (SCLC). Among these, the WEE1 inhibitor AZD1775 has shown clinical activity in a subset of SCLC patients, but resistance is common. Understanding primary and acquired resistance mechanisms will be critical for developing effective WEE1 inhibitor combinations.Experimental Design: AZD1775 sensitivity in SCLC cell lines was correlated with baseline expression level of 200 total or phosphorylated proteins measured by reverse-phase protein array (RPPA) to identify predictive markers of primary resistance. We further established AZD1775 acquired resistance models to identify mechanism of acquired resistance. Combination regimens were tested to overcome primary and acquired resistance to AZD1775 in in vitro and in vivo SCLC models.Results: High-throughput proteomic profiling demonstrate that SCLC models with primary resistance to AZD1775 express high levels of AXL and phosphorylated S6 and that WEE1/AXL or WEE1/mTOR inhibitor combinations overcome resistance in vitro and in vivo Furthermore, AXL, independently and via mTOR, activates the ERK pathway, leading to recruitment and activation of another G2-checkpoint protein, CHK1. AZD1775 acquired resistance models demonstrated upregulation of AXL, pS6, and MET, and resistance was overcome with the addition of AXL (TP0903), dual-AXL/MET (cabozantinib), or mTOR (RAD001) inhibitors.Conclusions: AXL promotes resistance to WEE1 inhibition via downstream mTOR signaling and resulting activation of a parallel DNA damage repair pathway, CHK1. These findings suggest rational combinations to enhance the clinical efficacy of AZD1775, which is currently in clinical trials for SCLC and other malignancies. Clin Cancer Res; 23(20); 6239-53. ©2017 AACR.

Selective Targeting of RSK Isoforms in Cancer

The p90 ribosomal S6 kinase family (RSK1-4) is a group of highly conserved Ser/Thr kinases that act as downstream effectors of the Ras/Raf/MEK/ERK signaling pathway. The RSKs phosphorylate a range of substrates involved in transcription, translation, cell cycle regulation, and cell survival. Although the RSKs have a high degree of sequence homology, their functional differences in cancer are of great interest. Current RSK inhibitors target more than one RSK isoform, and this may limit their efficacy as anticancer agents. Here, we review the structure and function of the RSK kinases, their role in cancer growth and survival, and their potential as modulators of chemoresistance. In addition, we summarize the development of current RSK inhibitors and their limitations.

The G2 checkpoint-a node-based molecular switch

Tight regulation of the eukaryotic cell cycle is paramount to ensure genomic integrity throughout life. Cell cycle checkpoints are present in each phase of the cell cycle and prevent cell cycle progression when genomic integrity is compromised. The G2 checkpoint is an intricate signaling network that regulates the progression of G2 to mitosis (M). We propose here a node-based model of G2 checkpoint regulation, in which the action of the central CDK1-cyclin B1 node is determined by the concerted but opposing activities of the Wee1 and cell division control protein 25C (CDC25C) nodes. Phosphorylation of both Wee1 and CDC25C at specific sites determines their subcellular localization, driving them either toward activity within the nucleus or to the cytoplasm and subsequent ubiquitin-mediated proteasomal degradation. In turn, this subcellular balance of the Wee1 and CDC25C nodes is directed by the action of the PLK1 and CHK1 nodes via what we have termed the 'nuclear and cytoplasmic decision states' of Wee1 and CDC25C. The proposed node-based model provides an intelligible structure of the complex interactions that govern the decision to delay or continue G2/M progression. The model may also aid in predicting the effects of agents that target these G2 checkpoint nodes.

Ras-activated RSK1 phosphorylates EBP50 to regulate its nuclear localization and promote cell proliferation

Differential subcellular localization of EBP50 leads to its controversial role in cancer biology either as a tumor suppressor when it resides at the membrane periphery, or a tumor facilitator at the nucleus. However, the mechanism behind nuclear localization of EBP50 remains unclear. A RNA interference screening identified the downstream effector of the Ras-ERK cascade, RSK1, as the molecule unique for nuclear transport of EBP50. RSK1 binds to EBP50 and phosphorylates it at a conserved threonine residue at position 156 (T156) under the regulation of growth factor. Mutagenesis experiments confirmed the significance of T156 residue in nuclear localization of EBP50, cellular proliferation, and oncogenic transformation. Our study sheds light on a possible therapeutic strategy targeting at this aberrant nuclear expression of EBP50 without affecting the normal physiological function of EBP50 at other subcellular localization.

Ndel1 suppresses ciliogenesis in proliferating cells by regulating the trichoplein-Aurora A pathway

Ndel1, a protein located at the subdistal appendage of mother centriole, functions as an upstream regulator of the trichoplein–Aurora A pathway that suppresses ciliogenesis in proliferating cells.

Primary cilia protrude from the surface of quiescent cells and disassemble at cell cycle reentry. We previously showed that ciliary reassembly is suppressed by trichoplein-mediated Aurora A activation pathway in growing cells. Here, we report that Ndel1, a well-known modulator of dynein activity, localizes at the subdistal appendage of the mother centriole, which nucleates a primary cilium. In the presence of serum, Ndel1 depletion reduces trichoplein at the mother centriole and induces unscheduled primary cilia formation, which is reverted by forced trichoplein expression or coknockdown of KCTD17 (an E3 ligase component protein for trichoplein). Serum starvation induced transient Ndel1 degradation, subsequent to the disappearance of trichoplein at the mother centriole. Forced expression of Ndel1 suppressed trichoplein degradation and axonemal microtubule extension during ciliogenesis, similar to trichoplein induction or KCTD17 knockdown. Most importantly, the proportion of ciliated and quiescent cells was increased in the kidney tubular epithelia of newborn Ndel1-hypomorphic mice. Thus, Ndel1 acts as a novel upstream regulator of the trichoplein–Aurora A pathway to inhibit primary cilia assembly.

Evidence for a role of the histone deacetylase SIRT6 in DNA damage response of multiple myeloma cells

Multiple myeloma (MM) is characterized by a highly unstable genome, with aneuploidy observed in nearly all patients. The mechanism causing this karyotypic instability is largely unknown, but recent observations have correlated these abnormalities with dysfunctional DNA damage response. Here, we show that the NAD(+)-dependent deacetylase SIRT6 is highly expressed in MM cells, as an adaptive response to genomic stability, and that high SIRT6 levels are associated with adverse prognosis. Mechanistically, SIRT6 interacts with the transcription factor ELK1 and with the ERK signaling-related gene. By binding to their promoters and deacetylating H3K9 at these sites, SIRT6 downregulates the expression of mitogen-activated protein kinase (MAPK) pathway genes, MAPK signaling, and proliferation. In addition, inactivation of ERK2/p90RSK signaling triggered by high SIRT6 levels increases DNA repair via Chk1 and confers resistance to DNA damage. Using genetic and biochemical studies in vitro and in human MM xenograft models, we show that SIRT6 depletion both enhances proliferation and confers sensitization to DNA-damaging agents. Our findings therefore provide insights into the functional interplay between SIRT6 and DNA repair mechanisms, with implications for both tumorigenesis and the treatment of MM.

A novel mouse model for ataxia-telangiectasia with a N-terminal mutation displays a behavioral defect and a low incidence of lymphoma but no increased oxidative burden

Ataxia-telangiectasia (A-T) is a rare multi-system disorder caused by mutations in the ATM gene. Significant heterogeneity exists in the underlying genetic mutations and clinical phenotypes. A number of mouse models have been generated that harbor mutations in the distal region of the gene, and a recent study suggests the presence of residual ATM protein in the brain of one such model. These mice recapitulate many of the characteristics of A-T seen in humans, with the notable exception of neurodegeneration. In order to study how an N-terminal mutation affects the disease phenotype, we generated an inducible Atm mutant mouse model (Atm(tm1Mmpl/tm1Mmpl), referred to as A-T [M]) predicted to express only the first 62 amino acids of Atm. Cells derived from A-T [M] mutant mice exhibited reduced cellular proliferation and an altered DNA damage response, but surprisingly, showed no evidence of an oxidative imbalance. Examination of the A-T [M] animals revealed an altered immunophenotype consistent with A-T. In contrast to mice harboring C-terminal Atm mutations that disproportionately develop thymic lymphomas, A-T [M] mice developed lymphoma at a similar rate as human A-T patients. Morphological analyses of A-T [M] cerebella revealed no substantial cellular defects, similar to other models of A-T, although mice display behavioral defects consistent with cerebellar dysfunction. Overall, these results suggest that loss of Atm is not necessarily associated with an oxidized phenotype as has been previously proposed and that loss of ATM protein is not sufficient to induce cerebellar degeneration in mice.

Crucial roles of RSK in cell motility by catalysing serine phosphorylation of EphA2

Crosstalk between inflammatory signalling pathways and receptor tyrosine kinases has been revealed as an indicator of cancer malignant progression. In the present study, we focus on EphA2 receptor tyrosine kinase, which is overexpressed in many human cancers. It has been reported that ligand-independent phosphorylation of EphA2 at Ser-897 is induced by Akt. We show that inflammatory cytokines promote RSK-, not Akt-, dependent phosphorylation of EphA2 at Ser-897. In addition, the RSK-EphA2 signalling pathway controls cell migration and invasion of metastatic breast cancer cells. Moreover, Ser-897-phosphorylated EphA2 co-localizes with phosphorylated active form of RSK in various human tumour specimens, and this double positivity is related to poor survival in lung cancer patients, especially those with a smoking history. Taken together, these results indicate that the phosphorylation of EphA2 at Ser-897 is controlled by RSK and the RSK-EphA2 axis might contribute to cell motility and promote tumour malignant progression.

Oncogenic BRAF(V600E) Induces Clastogenesis and UVB Hypersensitivity

The oncogenic BRAF(V600E) mutation is common in melanomas as well as moles. The roles that this mutation plays in the early events in the development of melanoma are poorly understood. This study demonstrates that expression of BRAF(V600E) is not only clastogenic, but synergizes for clastogenesis caused by exposure to ultraviolet radiation in the 300 to 320 nM (UVB) range. Expression of BRAF(V600E) was associated with induction of Chk1 pS280 and a reduction in chromatin remodeling factors BRG1 and BAF180. These alterations in the Chk1 signaling pathway and SWI/SNF chromatin remodeling pathway may contribute to the clastogenesis and UVB sensitivity. These results emphasize the importance of preventing sunburns in children with developing moles.

Cytokinetic Failure-induced Tetraploidy Develops into Aneuploidy, Triggering Skin Aging in Phosphovimentin-deficient Mice

Tetraploidy, a state in which cells have doubled chromosomal sets, is observed in ∼20% of solid tumors and is considered to frequently precede aneuploidy in carcinogenesis. Tetraploidy is also detected during terminal differentiation and represents a hallmark of aging. Most tetraploid cultured cells are arrested by p53 stabilization. However, the fate of tetraploid cells in vivo remains largely unknown. Here, we analyze the ability to repair wounds in the skin of phosphovimentin-deficient (VIM^SA/SA) mice. Early into wound healing, subcutaneous fibroblasts failed to undergo cytokinesis, resulting in binucleate tetraploidy. Accordingly, the mRNA level of p21 (a p53-responsive gene) was elevated in a VIM^SA/SA-specific manner. Disappearance of tetraploidy coincided with an increase in aneuploidy. Thereafter, senescence-related markers were significantly elevated in VIM^SA/SA mice. Because our tetraploidy-prone mouse model also exhibited subcutaneous fat loss at the age of 14 months, another premature aging phenotype, our data suggest that following cytokinetic failure, a subset of tetraploid cells enters a new cell cycle and develops into aneuploid cells in vivo, which promote premature aging.

Novel insights into Chk1 regulation by phosphorylation

Checkpoint kinase 1 (Chk1) is a conserved protein kinase central to the cell-cycle checkpoint during DNA damage response (DDR). Until recently, ATR, a protein kinase activated in response to DNA damage or stalled replication, has been considered as the sole regulator of Chk1. Recent progress, however, has led to the identification of additional protein kinases involved in Chk1 phosphorylation, affecting the subcellular localization and binding partners of Chk1. In fact, spatio-temporal regulation of Chk1 is of critical importance not only in the DDR but also in normal cell-cycle progression. In due course, many potent inhibitors targeted to Chk1 have been developed as anticancer agents and some of these inhibitors are currently in clinical trials. In this review, we summarize the current knowledge of Chk1 regulation by phosphorylation.

The fork and the kinase: a DNA replication tale from a CHK1 perspective

Replication fork progression is being continuously hampered by exogenously introduced and naturally occurring DNA lesions and other physical obstacles. Checkpoint kinase 1 (Chk1) is activated at replication forks that encounter damaged DNA. Subsequently, Chk1 inhibits the initiation of new replication factories and stimulates the firing of dormant origins (those in the vicinity of stalled forks). Chk1 also avoids fork collapse into DSBs (double strand breaks) and promotes fork elongation. At the molecular level, the current model considers stalled forks as the site of Chk1 activation and the nucleoplasm as the location where Chk1 phosphorylates target proteins. This model certainly serves to explain how Chk1 modulates origin firing, but how Chk1 controls the fate of stalled forks is less clear. Interestingly, recent reports demonstrating that Chk1 phosphorylates chromatin-bound proteins and even holds kinase-independent functions might shed light on how Chk1 contributes to the elongation of damaged DNA. Indeed, such findings have unveiled a puzzling connection between Chk1 and DNA lesion bypass, which might be central to promoting fork elongation and checkpoint attenuation. In summary, Chk1 is a multifaceted and versatile signaling factor that acts at ongoing forks and replication origins to determine the extent and quality of the cellular response to replication stress.

Targeting CHK1 inhibits cell proliferation in FLT3-ITD positive acute myeloid leukemia

CHK1 Ser/Thr kinase, a well characterized regulator of DNA damage response, is also involved in normal cell cycle progression. In this study, we investigate how CHK1 participates to proliferation of acute myeloid leukemia cells expressing the mutated FLT3-ITD tyrosine kinase receptor. Pharmacological inhibition of CHK1 as well as its shRNA mediated down regulation reduced the proliferation rate of FLT-ITD expressing leukemic cell lines in a cytostatic manner. Flow cytometry analysis revealed no accumulation in a specific phase of the cell cycle upon CHK1 inhibition. Accordingly, lentiviral-mediated CHK1 overexpression increased the proliferation rate of FLT3-ITD expressing cells, as judged by cell viability and [3H] thymidine incorporation experiments. By contrast, expression of a ser280 mutant did not, suggesting that phosphorylation of this residue is an important determinant of CHK1 proliferative function. Clonogenic growth of primary leukemic cells from patients in semi-solid medium was reduced upon CHK1 inhibition, confirming the data obtained with leukemic established cell lines. Surprisingly, 3 out of 4 CHK1 inhibitory compounds tested in this study were also potent inhibitors of the FLT3-ITD tyrosine kinase receptor. Altogether, these data identify CHK1 as a regulator of FLT3-ITD-positive leukemic cells proliferation, and they open interesting perspectives in terms of new therapeutic strategies for these pathologies.

Wild-type H- and N-Ras promote mutant K-Ras-driven tumorigenesis by modulating the DNA damage response

Mutations in KRAS are prevalent in human cancers and universally predictive of resistance to anticancer therapeutics. Although it is widely accepted that acquisition of an activating mutation endows RAS genes with functional autonomy, recent studies suggest that the wild-type forms of Ras may contribute to mutant Ras-driven tumorigenesis. Here, we show that downregulation of wild-type H-Ras or N-Ras in mutant K-Ras cancer cells leads to hyperactivation of the Erk/p90RSK and PI3K/Akt pathways and, consequently, the phosphorylation of Chk1 at an inhibitory site, Ser 280. The resulting inhibition of ATR/Chk1 signaling abrogates the activation of the G2 DNA damage checkpoint and confers specific sensitization of mutant K-Ras cancer cells to DNA damage chemotherapeutic agents in vitro and in vivo.

Suppression of DNA-damage checkpoint signaling by Rsk-mediated phosphorylation of Mre11

Ataxia telangiectasia mutant (ATM) is an S/T-Q-directed kinase that is critical for the cellular response to double-stranded breaks (DSBs) in DNA. Following DNA damage, ATM is activated and recruited by the MRN protein complex [meiotic recombination 11 (Mre11)/DNA repair protein Rad50/Nijmegen breakage syndrome 1 proteins] to sites of DNA damage where ATM phosphorylates multiple substrates to trigger cell-cycle arrest. In cancer cells, this regulation may be faulty, and cell division may proceed even in the presence of damaged DNA. We show here that the ribosomal s6 kinase (Rsk), often elevated in cancers, can suppress DSB-induced ATM activation in both Xenopus egg extracts and human tumor cell lines. In analyzing each step in ATM activation, we have found that Rsk targets loading of MRN complex components onto DNA at DSB sites. Rsk can phosphorylate the Mre11 protein directly at S676 both in vitro and in intact cells and thereby can inhibit the binding of Mre11 to DNA with DSBs. Accordingly, mutation of S676 to Ala can reverse inhibition of the response to DSBs by Rsk. Collectively, these data point to Mre11 as an important locus of Rsk-mediated checkpoint inhibition acting upstream of ATM activation.

Inhibition of the PI3K/Akt/GSK3 pathway downstream of BCR/ABL, Jak2-V617F, or FLT3-ITD downregulates DNA damage-induced Chk1 activation as well as G2/M arrest and prominently enhances induction of apoptosis

Constitutively-activated tyrosine kinase mutants, such as BCR/ABL, FLT3-ITD, and Jak2-V617F, play important roles in pathogenesis of hematopoietic malignancies and in acquisition of therapy resistance. We previously found that hematopoietic cytokines enhance activation of the checkpoint kinase Chk1 in DNA-damaged hematopoietic cells by inactivating GSK3 through the PI3K/Akt signaling pathway to inhibit apoptosis. Here we examine the possibility that the kinase mutants may also protect DNA-damaged cells by enhancing Chk1 activation. In cells expressing BCR/ABL, FLT3-ITD, or Jak2-V617F, etoposide induced a sustained activation of Chk1, thus leading to the G2/M arrest of cells. Inhibition of these kinases by their inhibitors, imatinib, sorafenib, or JakI-1, significantly abbreviated Chk1 activation, and drastically enhanced apoptosis induced by etoposide. The PI3K inhibitor GD-0941 or the Akt inhibitor MK-2206 showed similar effects with imatinib on etoposide-treated BCR/ABL-expressing cells, including those expressing the imatinib-resistant T315I mutant, while expression of the constitutively activated Akt1-myr mutant conferred resistance to the combined treatment of etoposide and imatinib. GSK3 inhibitors, including LiCl and SB216763, restored the sustained Chk1 activation and mitigated apoptosis in cells treated with etoposide and the inhibitors for aberrant kinases, PI3K, or Akt. These observations raise a possilibity that the aberrant kinases BCR/ABL, FLT3-ITD, and Jak2-V617F may prevent apoptosis induced by DNA-damaging chemotherapeutics, at least partly through enhancement of the Chk1-mediated G2/M checkpoint activation, by inactivating GSK3 through the PI3K/Akt signaling pathway. These results shed light on the molecular mechanisms for chemoresistance of hematological malignancies and provide a rationale for the combined treatment with chemotherapy and the tyrosine kinase or PI3K/Akt pathway inhibitors against these diseases.

The p90 ribosomal S6 kinase (RSK) inhibitor BI-D1870 prevents gamma irradiation-induced apoptosis and mediates senescence via RSK- and p53-independent accumulation of p21WAF1/CIP1

The p90 ribosomal S6 kinase (RSK) family is a group of highly conserved Ser/Thr kinases that promote cell proliferation, growth, motility and survival. As they are almost exclusively activated downstream of extracellular signal-regulated kinases 1 and 2 (ERK1/2), therapeutic intervention by RSK inhibition is less likely to produce such severe side effects as those observed following inhibition of the upstream master regulators Raf, MEK and ERK1/2. Here, we report that BI-D1870, a potent small molecule inhibitor of RSKs, induces apoptosis, although preferentially, in a p21-deficient background. On the other hand, BI-D1870 also induces a strong transcription- and p53-independent accumulation of p21 protein and protects cells from gamma irradiation (γIR)-induced apoptosis, driving them into senescence even in the absence of γIR. Although we identified p21 in in vitro kinase assays as a novel RSK substrate that specifically becomes phosphorylated by RSK1-3 at Ser116 and Ser146, RNA-interference, overexpression and co-immunoprecipitation studies as well as the use of SL0101, another specific RSK inhibitor, revealed that BI-D1870 mediates p21 accumulation via a yet unknown pathway that, besides its off-site targets polo-like kinase-1 and AuroraB, also does also not involve RSKs. Thus, this novel off-target effect of BI-D1870 should be taken into serious consideration in future studies investigating the role of RSKs in cellular signaling and tumorigenesis.

Axl, a prognostic and therapeutic target in acute myeloid leukemia mediates paracrine crosstalk of leukemia cells with bone marrow stroma

Acute myeloid leukemia (AML) represents a clonal disease of hematopoietic progenitors characterized by acquired heterogenous genetic changes that alter normal mechanisms of proliferation, self-renewal, and differentiation.(1) Although 40% to 45% of patients younger than 65 years of age can be cured with current therapies, only 10% of older patients reach long-term survival.(1) Because only very few novel AML drugs were approved in the past 2 decades, there is an urgent need to identify novel targets and therapeutic strategies to treat underserved AML patients. We report here that Axl, a member of the Tyro3, Axl, Mer receptor tyrosine kinase family,(2-4) represents an independent prognostic marker and therapeutic target in AML. AML cells induce expression and secretion of the Axl ligand growth arrest-specific gene 6 (Gas6) by bone marrow-derived stromal cells (BMDSCs). Gas6 in turn mediates proliferation, survival, and chemoresistance of Axl-expressing AML cells. This Gas6-Axl paracrine axis between AML cells and BMDSCs establishes a chemoprotective tumor cell niche that can be abrogated by Axl-targeting approaches. Axl inhibition is active in FLT3-mutated and FLT3 wild-type AML, improves clinically relevant end points, and its efficacy depends on presence of Gas6 and Axl. Axl inhibition alone or in combination with chemotherapy might represent a novel therapeutic avenue for AML.

Pim kinases phosphorylate Chk1 and regulate its functions in acute myeloid leukemia

Phosphorylation by Akt on Ser 280 was reported to induce cytoplasmic retention and inactivation of CHK1 with consequent genetic instability in PTEN-/- cells. In acute myeloid leukemia cells carrying the FLT3-internal tandem duplication (ITD) mutation, we observed high rates of FLT3-ITD-dependent CHK1 Ser 280 phosphorylation. Pharmacological inhibition and RNA interference identified Pim1/2, not Akt, as effectors of this phosphorylation. Pim1 catalyzed Ser 280 phosphorylation in vitro and ectopic expression of Pim1/2-induced CHK1 phosphorylation. Ser 280 phosphorylation did not modify CHK1 localization, but facilitated its cell cycle and resistance functions in leukemic cells. FLT3, PIM or CHK1 inhibitors synergized with DNA-damaging agents to induce apoptosis, allowing cells to bypass the etoposide-induced G2/M arrest. Consistently, etoposide-induced CHK1-dependent phosphorylations of CDC25C on Ser 216 and histone H3 on Thr11 were decreased upon FLT3 inhibition. Accordingly, ectopic expression of CHK1 improved the resistance of FLT3-ITD cells and maintained histone H3 phosphorylation in response to DNA damage, whereas expression of unphosphorylated Ser 280Ala mutant did not. Finally, FLT3- and Pim-dependent phosphorylation of CHK1 on Ser 280 was confirmed in primary blasts from patients. These results identify a new pathway involved in the resistance of FLT3-ITD leukemic cells to genotoxic agents, and they constitute the first report of CHK1 Ser 280 regulation in myeloid malignancies.

Roles of Chk1 in cell biology and cancer therapy

The evolutionally conserved DNA damage response (DDR) and cell cycle checkpoints preserve genome integrity. Central to these genome surveillance pathways is a protein kinase, Chk1. DNA damage induces activation of Chk1, which then transduces the checkpoint signal and facilitates cell cycle arrest and DNA damage repair. Significant progress has been made recently toward our understanding of Chk1 regulation and its implications in cancer etiology and therapy. Specifically, a model that involves both spatiotemporal and conformational changes of proteins has been proposed for Chk1 activation. Further, emerging evidence suggests that Chk1 does not appear to be a tumor suppressor; instead, it promotes tumor growth and may contribute to anticancer therapy resistance. Recent data from our laboratory suggest that activating, but not inhibiting, Chk1 in the absence of chemotherapy might represent an innovative approach to suppress tumor growth. These findings suggest unique regulation of Chk1 in cell biology and cancer etiology, pointing to novel strategies for targeting Chk1 in cancer therapy.

Candidate tumor suppressor BTG3 maintains genomic stability by promoting Lys63-linked ubiquitination and activation of the checkpoint kinase CHK1

B-cell translocation gene 3 (BTG3) is a member of the antiproliferative BTG/ Transducer of ErbB2 gene family and is induced by genotoxic stress in a p53- and Checkpoint kinase 1 (CHK1)-dependent manner. Down-regulation of BTG3 has been observed in human cancers, suggesting that it plays an important role in tumor suppression, although the underlying mechanisms are unclear. Here, we report that BTG3 interacts with CHK1, a key effector kinase in the cell cycle checkpoint response, and regulates its phosphorylation and activation. Upon interaction, BTG3 mediates K63-linked ubiquitination of CHK1 at Lys132 through the cullin-RING ligase 4(Cdt2) E3 complex, thus facilitating CHK1 chromatin association. We show that BTG3-depleted cells phenocopy those CHK1-deficient cells, exhibiting increased cell death after replication block and impaired chromosome alignment and segregation. These defects could be corrected by wild-type BTG3 but not by a mutant impaired in CHK1 interaction. We propose that BTG3-dependent CHK1 ubiquitination contributes to its chromatin localization and activation and that a defect in this regulation may increase genome instability and promote tumorigenesis.

PI 3-kinase-dependent phosphorylation of Plk1-Ser99 promotes association with 14-3-3γ and is required for metaphase-anaphase transition

Polo-like kinase 1 (Plk1) controls multiple aspects of mitosis and is activated through its phosphorylation at Thr210. Here we identify Ser99 on Plk1 as a novel mitosis-specific phosphorylation site, which operates independently of Plk1-Thr210 phosphorylation. Plk1-Ser99 phosphorylation creates a docking site for 14-3-3γ, and this interaction stimulates the catalytic activity of Plk1. Knockdown of 14-3-3γ or replacement of wild-type (WT) Plk1 by a Ser99-phospho-blocking mutant leads to a prometaphase/metaphase-like arrest due to the activation of the spindle assembly checkpoint. Inhibition of phosphatidylinositol 3-kinase (PI3K) and Akt significantly reduces the level of Plk1-Ser99 phosphorylation and delays metaphase to anaphase transition. Plk1-Ser99 phosphorylation requires not only Akt activity but also protein(s) associated with Plk1 in a mitosis-specific manner. Therefore, mitotic Plk1 activity is regulated not only by Plk1-Thr210 phosphorylation, but also by Plk1 binding to 14-3-3γ following Plk1-Ser99 phosphorylation downstream of the PI3K-Akt signalling pathway. This novel Plk1 activation pathway controls proper progression from metaphase to anaphase.

RSK promotes G2 DNA damage checkpoint silencing and participates in melanoma chemoresistance

The incidence of malignant melanoma is growing rapidly worldwide and there is still no effective therapy for metastatic disease. This type of cancer is highly resistant to conventional DNA-damaging chemotherapeutics, and intense research has been dedicated for understanding the molecular pathways underlying chemoresistance. The Ras/mitogen-activated protein kinase (MAPK) signalling pathway is often deregulated in melanoma, which frequently harbours activating mutations in NRAS or BRAF. Herein, we demonstrate that the MAPK-activated protein kinase RSK (p90 ribosomal S6 kinase) contributes to melanoma chemoresistance by altering their response to chemotherapeutic agents. We find that RSK phosphorylates checkpoint kinase 1 (Chk1) at an inhibitory site, Ser280, both in vitro and in vivo. Our results indicate that RSK is the predominant protein kinase operating downstream of mitogens and oncogenes of the Ras/MAPK pathway, and consistent with this, we find that RSK constitutively phosphorylates Chk1 in melanoma. We show that RSK inhibition increases Chk1 activity in response to DNA-damaging agents, suggesting that the Ras/MAPK pathway modulates Chk1 function and the response to DNA damage. Accordingly, we demonstrate that RSK promotes G2 DNA damage checkpoint silencing in a Chk1-dependent manner, and find that RSK inhibitors sensitize melanoma cells to DNA-damaging agents. Together, our results identify a novel link between the Ras/MAPK pathway and the DNA damage response, and suggest that RSK inhibitors may be used to modulate chemosensitivity, which is one of the major obstacles to melanoma treatment.

Novel regulation of checkpoint kinase 1: Is checkpoint kinase 1 a good candidate for anti-cancer therapy?

DNA-damaging strategies, such as radiotherapy and the majority of chemotherapeutic therapies, are the most frequently used non-surgical anti-cancer therapies for human cancers. These therapies activate DNA damage/replication checkpoints, which induce cell-cycle arrest to provide the time needed to repair DNA damage. Due to genetic defect(s) in the ATM (ataxia-telangiectasia mutated)-Chk2-p53 pathway, an ATR (ATM- and Rad3-related)-Chk1-Cdc25 route is the sole checkpoint pathway in a majority of cancer cells. Chk1 inhibitors are expected to selectively induce the mitotic cell death (mitotic catastrophe) of cancer cells. However, recent new findings have pointed out that Chk1 is essential for the maintenance of genome integrity even during unperturbed cell-cycle progression, which is controlled by a variety of protein kinases. These observations have raised concerns about a possible risk of Chk1 inhibitors on the clinics. In this review, we summarize recent advances in Chk1 regulation by phosphorylation, and discuss Chk1 as a molecular target for cancer therapeutics.